Merge pull request swiftlang#31638 from compnerd/discarded-dependencies

runtime: add and switch to `SWIFT_NODISCARD` (NFC)

Author: compnerd

stdlib/public/SwiftShims/RefCount.h

@@ -546,7 +546,7 @@ class RefCountBitsT {
   // Returns true if the increment is a fast-path result.
   // Returns false if the increment should fall back to some slow path
   // (for example, because UseSlowRC is set or because the refcount overflowed).
-  LLVM_NODISCARD SWIFT_ALWAYS_INLINE bool
+  SWIFT_NODISCARD SWIFT_ALWAYS_INLINE bool
   incrementStrongExtraRefCount(uint32_t inc) {
     // This deliberately overflows into the UseSlowRC field.
     bits += BitsType(inc) << Offsets::StrongExtraRefCountShift;
@@ -557,7 +557,7 @@ class RefCountBitsT {
   // Returns false if the decrement should fall back to some slow path
   // (for example, because UseSlowRC is set
   // or because the refcount is now zero and should deinit).
-  LLVM_NODISCARD SWIFT_ALWAYS_INLINE bool
+  SWIFT_NODISCARD SWIFT_ALWAYS_INLINE bool
   decrementStrongExtraRefCount(uint32_t dec) {
 #ifndef NDEBUG
     if (!hasSideTable() && !isImmortal(false)) {
@@ -1409,8 +1409,8 @@ class HeapObjectSideTableEntry {
 
 
   // WEAK
-  
-  LLVM_NODISCARD
+
+  SWIFT_NODISCARD
   HeapObjectSideTableEntry* incrementWeak() {
     // incrementWeak need not be atomic w.r.t. concurrent deinit initiation.
     // The client can't actually get a reference to the object without

stdlib/public/SwiftShims/Visibility.h

@@ -149,6 +149,14 @@
 #define SWIFT_FALLTHROUGH
 #endif
 
+#if __cplusplus >= 201402l && __has_cpp_attribute(nodiscard)
+#define SWIFT_NODISCARD [[nodiscard]]
+#elif __has_cpp_attribute(clang::warn_unused_result)
+#define SWIFT_NODISCARD [[clang::warn_unused_result]]
+#else
+#define SWIFT_NODISCARD
+#endif
+
 
 /// Attributes for runtime-stdlib interfaces.
 /// Use these for C implementations that are imported into Swift via SwiftShims